1. Field of the Invention
The present invention relates generally to a broadband wireless access communication system, and more particularly to a method for measuring and reporting a channel quality in a broadband wireless access communication system for use with an OFDM (Orthogonal Frequency Division Multiplexing) scheme.
2. Description of the Related Art
A great deal of intensive research has been conducted on the 4G (4th Generation) communication system as one of the next generation communication systems to provide a plurality of users with a specific service having a variety of QoSs (Quality of Services) at a transfer rate of about 100 Mbps. Presently, the 3G (3rd Generation) communication system provides a transfer rate of about 384 kbps in an outdoor channel environment having a relatively poor channel environment, and provides a maximum transfer rate of about 2 Mbps in an indoor channel environment having a relatively good channel environment. A wireless Local Area Network (LAN) system and a wireless Metropolitan Area Network (MAN) system have been designed to provide a transfer rate of 20˜50 Mbps. Further, a new communication system based on the 4G communication system has been developed to provide the wireless LAN and MAN systems for guaranteeing a relatively high transfer rate with mobility and QoS. As a result, many developers have conducted intensive research into a high-speed service to be provided from the 4G communication system.
However, the wireless MAN system is suitable for a high-speed communication service in that it has a wide coverage and supports a high-speed transfer rate, but it does not consider the mobility of a subscriber station (SS). Consequently, there is no consideration of a handover operation caused by the high-speed movement of the SS. The communication system currently considered in the IEEE (Institute of Electrical and Electronics Engineers) 802.16a specification acts as a specific communication system for performing a ranging operation between the SS and a base station (BS). FIG.
FIG. 1 is a block diagram illustrating a broadband wireless access communication system using an OFDM/OFDMA (Orthogonal Frequency Division Multiplexing/Orthogonal Frequency Division Multiple Access) scheme. More specifically. FIG. 1 depicts the IEEE 802.16a communication system.
The wireless MAN system acting as a BWA (Broadband Wireless Access) communication system has a much wider coverage and a much higher transfer rate than the wireless LAN system. When adapting the OFDM scheme and the OFDMA scheme to a physical channel of the wireless MAN system to provide the wireless MAN system with a broadband transmission network, this application system is called an IEEE 802.16a communication system. The IEEE 802.16a communication system applies the OFDM/OFDMA scheme to the wireless MAN system, such that it transmits a physical channel signal using a plurality of sub-carriers, resulting in high-speed data transmission.
The IEEE 802.16e communication system has been designed to consider the SS's mobility in the IEEE 802.16a communication system, and there is no detailed specification for the IEEE 802.16e communication system. The IEEE 802.16a communication system and the IEEE 802.16e communication system act as a broadband wireless access communication system for use with the OFDM/OFDMA schemes. For the convenience of description, the IEEE 802.16a communication system will be adapted as an example.
Referring to FIG. 1, the IEEE 802.16a communication system has a single cell structure, and comprises a BS 100 and a plurality of SSs 110, 120, and 130, which are managed by the BS 100. Signal transmission/reception among the BS 100 and the SSs 110, 120, and 130 can be established using the OFDM/OFDMA scheme. FIG.
FIG. 2 is a conceptual diagram illustrating the downlink frame structure for use in the BWA communication system using the OFDM/OFDMA scheme. More specifically, FIG. 2 depicts a downlink frame structure for use in the IEEE 802.16a/IEEE 802.16e communication system.
Referring to FIG. 2, the downlink frame includes a preamble field 200, a broadcast control field 210, and a plurality of TDM (Time Division Multiplexing) fields 220 and 230. A synchronous signal (i.e., a preamble sequence) for synchronizing the BS and the SSs is transmitted via the preamble field 200. The broadcast control field 210 includes a DL(DownLink)_MAP field 211 and a UL(UpLink)_MAP field 213. The DL_MAP field 211 transmits the DL_MAP message. A plurality of IEs (Information Elements) contained in the DL_MAP message are shown in Table 1 below.
TABLE 1SyntaxSizeNotesDL_MAP_Message_Format( ){Management Message Type=28 bitsPHY Synchronization FieldVariableSee Appropriate PHYspecificationDCD Count8 bitsBase Station ID48 bits Number of DL_MAP Element n16 bits Begin PHY Specific section {See ApplicablePHY section For (i=1; i<=n; i++)For each DL_MAPelement 1 to nDL_MAP Information Element( )VariableSee corresponding PHYspecification if!(byte boundary) {4 bitsPadding to reach  Padding Nibblebyte boundary     }    }   }  }
Referring to Table 1, the DL_MAP message includes a Management Message Type field including a plurality of IEs (i.e., transmission message type information); a PHY (PHYsical) Synchronization field established in response to a modulation or demodulation scheme applied to a physical channel in order to perform synchronization acquisition; a DCD count field including count information in response to a DCD (Downlink Channel Descript) message configuration variation containing a downlink burst profile; a Base Station ID field including a Base Station Identifier; and a Number of DL_MAP Element n field including the number of elements found after the Base Station ID. Particularly, the DL_MAP message (not shown in Table 1) includes information associated with ranging codes allocated to individual ranging processes to be described later.
The UL_MAP field 213 transmits the UL_MAP message. A plurality of IEs contained in the UL_MAP message are shown in Table 2 below.
TABLE 2SyntaxSizeNotesUL_MAP_Message_Format( ){Management Message Type=38 bitsUplink Channel ID8 bitsUCD Count8 bitsNumber of UL_MAP Element n16 bits Allocation Start Time32 bits Begin PHY Specific section {See ApplicablePHY section for (I=1; i<=n; i++)For each DL_MAPelement 1 to nUL_MAP Information_Element( )VariableSee corresponding PHYspecification     }    }   }
Referring to Table 2, the UL_MAP message includes a Management Message Type field including a plurality of IEs (i.e., transmission message type information); an Uplink Channel ID field including a used Uplink Channel ID; a UCD (Uplink Channel Descript) count field including count information in response to a UCD message configuration variation containing an uplink burst profile; and a Number of UL_MAP Element n field including the number of elements found after the UCD count field. In this case, the uplink channel ID can only be allocated to a Media Access Control (MAC) sub-layer.
The TDM fields 220 and 230 are timeslots using a TDM/TDMA (Time Division Multiple/Time Division Multiple Access) scheme. The BS transmits broadcast information to be broadcast to SSs managed by the BS over the DL_MAP field 211 using a predetermined center carrier. The SSs monitor all the frequency bands having been previously allocated to individual SSs upon receipt of a power-on signal, such that they detect a pilot channel signal having a highest signal intensity, i.e., the highest SINR (Signal to Interference and Noise Ratio). It is determined that the SS belongs to a specific BS, which has transmitted the pilot channel signal with the highest SINR. The SSs check the DL_MAP field 211 and the UL_MAP field 213 of the downlink frame transmitted from the BS, such that they recognize their own uplink and downlink control information and specific information for indicating a real data transmission/reception position.
The aforementioned UCD message configuration is shown in Table 3 below.
TABLE 3SyntaxSizeNotesUCD-Message_Format( ){Management Message Type=08 bitsUnlink channel ID8 bitsConfiguration Change Count8 bitsMini-slot size8 bitsRanging Backoff Start8 bitsRanging Backoff End8 bitsRequest Backoff Start8 bitsRequest Backoff End8 bitsTLV Encoded Information for theVariableoverall channelBegin PHY Specific Section { for (I=1; i<n; i+n)  Uplink_Burst_DescriptorVariable  } }}
Referring to Table 3, the UCD message includes a Management Message Type field including a plurality of IEs (i.e., transmission message type information); an Uplink Channel ID field including a used Uplink Channel Identifier; a Configuration Change Count field counted by the BS; a mini-slot size field including the size of the mini-slot of the uplink physical channel: a Ranging Backoff Start field including a backoff start point for an initial ranging process, i.e., an initial backoff window size for the initial ranging process; a Ranging Backoff End field including a backoff end point for the initial ranging process, i.e., a final backoff window size; a Request Backoff Start field including a backoff start point for establishing contention data and requests, i.e., an initial backoff window size; and a Request Backoff End field including a backoff end point for establishing contention data and requests, i.e., a final backoff window size. In this case, the backoff value indicates a kind of standby time which is a duration time between the start of SS's access failure and the start of SS's re-access time. If the SS fails to execute an initial ranging process, the BS must transmit the backoff values indicative of standby time information for which the SS must wait for the next ranging process to the SS. For example, provided that a specific number of 10 is determined by the “Ranging Backoff Start” and “Ranging Backoff End” fields shown in the Table 3. the SS must pass over 210 access executable chances (i.e., 1024 access executable chances) and then execute the next ranging process according to the Truncated Binary Exponential Backoff Algorithm.
FIG. 3 is a conceptual diagram illustrating an uplink frame structure for use in a BWA communication system using an OFDM/OFDMA scheme. More specifically, FIG. 3 depicts an uplink frame structure for use in the IEEE 802.16a communication system.
Prior to describing the uplink frame structure illustrated in FIG. 3, three ranging processes for use in the IEEE 802.16a communication system, i.e., an initial ranging process, a maintenance ranging process (also called a period ranging process), and a bandwidth request ranging process will hereinafter be described in detail.
The initial ranging process for establishing synchronization acquisition between the BS and the SS establishes a correct time offset between the SS and the BS, and controls a transmission power (also called a transmit power). More specifically, the SS is powered on, and receives the DL_MAP message, the UL_MAP message, and the UCD message to establish synchronization with the BS in such a way that it performs the initial ranging process to control the transmission power between the BS and the time offset. In this case, the IEEE 802.16a communication system uses the OFDM/OFDMA scheme, such that the ranging procedure requires a plurality of ranging sub-channels and a plurality of ranging codes. The BS allocates available ranging codes to the SS according to objectives of the ranging processes (i.e., the ranging process type information). This operation will hereinafter be described in more detail.
The ranging codes are created by segmenting a PN (Pseudorandom Noise) sequence having a length of 215−1 bits into predetermined units. Typically, one ranging channel is composed of two ranging sub-channels each having a length of 53 bits. PN code segmentation is executed over the ranging channel having the length of 106 bits, resulting in the creation of a ranging code. A maximum of 48 ranging codes RC#1˜RC#48 can be assigned to the SS. More than two ranging codes for every SS are applied as a default value to the three ranging processes having different objectives i.e., an initial ranging process, a period ranging process, and a bandwidth request ranging process. In this way, a ranging code is differently assigned to the SS according to each objective of the three ranging processes. For example, N ranging codes are assigned to the SS for the initial ranging process as denoted by a prescribed term of “N RC (Ranging Codes) for Initial Ranging”, M ranging codes are assigned to the SS for the periodic ranging process as denoted by a prescribed term of “M RCs for maintenance ranging”, and L ranging codes are assigned to the SS for the bandwidth request ranging process as denoted by a prescribed term of “L RCs for BW-request ranging”. The assigned ranging codes are transmitted to the SSs using the DL_MAP message, and the SSs perform necessary ranging procedures using the ranging codes contained in the DL_MAP message.
The period ranging process is periodically executed Such that an SS which has controlled a time offset between the SS and the BS and a transmission power in the initial ranging process can control a channel state associated with the BS. The SS performs the period ranging process using the ranging codes assigned for the period ranging process.
The bandwidth request ranging process enables the SS, which has controlled a time offset between the SS and the BS and a transmission power in the initial ranging process, to request a bandwidth allocation from the BS in such a way that the SS can communicate with the BS.
Referring to FIG. 3, the uplink frame includes an initial maintenance opportunity field 300 using the initial and period ranging processes, a request contention opportunity field 310 using the bandwidth request ranging process, and an SS scheduled data field 320 including uplink data of a plurality of SSs. The initial maintenance opportunity field 300 includes a plurality of access burst fields each having initial and period ranging processes, and a collision field in which there is a collision between the access burst fields. The request contention opportunity field 310 includes a plurality of bandwidth request fields each having a real bandwidth request ranging process, and a collision field in which there is a collision between the bandwidth request ranging fields. The SS scheduled data fields 320 each include a plurality of SS scheduled data fields (i.e., SS 1 scheduled data field˜SS N scheduled data field). The SS transition gap is positioned between the SS scheduled data fields (i.e., SS 1 scheduled data field˜SS N scheduled data field).
The UIUC (Uplink Interval Usage Code) area records information identifying the usage of offsets recorded in the offset area. For example, provided that 2 is recorded in the UIUC area, a starting offset for use in the initial ranging process is recorded in the offset area. When 3 is recorded in the UIUC area, a starting offset for use in either the bandwidth request ranging or the maintenance ranging process is recorded in the offset area. The offset area records a starting offset value for use in either the initial ranging process or the maintenance ranging process according to the information recorded in the UIUC area. Physical channel characteristic information to be transferred from the UIUC area is recorded in the UCD.
As described above, the IEEE 802.16a communication system has considered a fixed state of a current SS (i.e., there is no consideration given to the mobility of the SS) and a single cell structure. However, the IEEE 802.16e communication system has been defined as a system for considering the SS's mobility in the IEEE 802.16a communication system, such that the IEEE 802.16e communication system must consider the SS's mobility in a multi-cell environment. In order to provide the SS's mobility in the multi-cell environment, individual operations modes of the SS and the BS must be converted. More specifically, many developers have conducted intensive research into an SS handover system considering a multi-cell structure to provide the SS's mobility.
As such, in order to enable the IEEE 802.16e communication system to support a handover function, the SS must measure SINRs of pilot signals transferred from neighbor BSs and an active BS to which the SS currently belong. When the SINR of the pilot signal transferred from the active BS is lower than SINRs of pilot signals transferred from the neighbor BSs, the SS transmits a handover request to the active BS. A method for controlling a mobile SS to measure the SINRs of the pilot signals transferred from the active BS and the neighbor BSs in the IEEE 802.16e communication system will be described later in more detail with reference to FIG. 4. In this case, the expression “Pilot signal's SINR measurement” is called a “Pilot signal's SINR scan or scanning” for the convenience of description. It should be noted that the term “Scan” is substantially equal to the other term “Scanning”.
FIG. 4 is a flow chart illustrating a method for measuring SINRs of pilot signals transferred from the active BS and the neighbor BSs in a broadband wireless access communication system for use with a conventional OFDM/OFDMA scheme. More specifically, the method for measuring SINRs of pilot signals transferred from the active BS and the neighbor BSs in the IEEE 802.16e communication system is illustrated in FIG. 4.
However, prior to describing FIG. 4, as indicated above, the IEEE 802.16e communication system considers the mobility of SSs in the IEEE 802.16a communication system. The SS with the mobility in the IEEE 802.16e communication system is called an MSS (Mobile Subscriber Station).
Referring to FIG. 4, the BS 450 transmits an NBR_ADV (Neighbor BSs Advertisement) message to the MSS 400 at step 411. The detailed configuration of the NBR_ADV message is shown in Table 4 below.
TABLE 4SyntaxSizeNotesNBR_ADV Message_Format( ){ Management Message Type=?8 bits N_NEIGHBORS8 bits For(i=0;j<N_NEIGHBORS;j++){  Neighbor BS-ID48 bits   Configuration Change Count8 bits  Physical Frequency16 bits   TLV Encoded Neighbor InformationVariableTLV specific  }}
Referring to Table 4, the NBR_ADV message includes a Management Message Type field including transmission message type information; an N_NEIGHBORS field including the number of neighbor BSs; a neighbor BS-ID field including ID information of the neighbor BSs; a Configuration Change Count field including the number of configuration changes; a physical frequency field including physical channel frequencies of the neighbor BSs: and a TLV (Type/Length/Value) Encoded Neighbor Information field including other information associated with neighbor BSs other than the above described information. It should be noted that the Management Message Type field to which the NBR_ADV message will be transmitted is currently in an undecided state, as denoted by “Management Message Type=? (undecided)”.
The MSS 400 that is receiving the NBR_ADV message transmits a SCAN_REQ (Scan Request) message to the BS 450 when it wishes to scan SINRs of pilot signals transferred from the neighbor BSs at step 413. In this case, the time at which the MSS 400 generates a scan request is not directly associated with the pilot SINR scanning operation, such that its detailed description will herein be omitted.
The SCAN_REQ message configuration is shown in Table 5 below.
TABLE 5SyntaxSizeNotesSCN_REQ Message_Format( ){ Management Message Type=? 8 bits Scan Duration20 bitsFor SCa PHY, units aremini-slots.For OFDM/OFDMA PHY,units are OFDMsymbols}
Referring to Table 5, the SCAN_REQ message includes a Management Message Type field including a plurality of IEs (i.e., transmission message type information), and a Scan Duration field including a scan-desired scan duration for SINRs of pilot signals transferred from the neighbor BSs. If the IEEE 802.16e communication system is a system for use with a Single Carrier (SC), i.e., if the scan duration field is applied to an SC physical channel, the scan duration field is configured in units of mini-slots. If the IEEE 802.16e system acts as the OFDM/OFDMA system, i.e., if the IEEE 802.16e system is applied to the OFDM/OFDMA physical channel it is configured in the form of OFDM-symbol units. It should be noted that the Management Message Type field to which the SCAN REQ message will be transmitted is currently in an undecided state, as denoted by “Management Message Type=? (undecided)”.
The BS 450 receiving the SCAN_REQ message transmits a DL_MAP message including information to be scanned by the MSS 400 to the MSS 400 at step 415. In this case, the SCANNING_IE message including scan information contained in the DL_MAP message is shown in Tables 6, 7. and 8 below.
TABLE 6For SCa PHY:SyntaxSizeNotesScanning_IE { CID16 bitsMSS basic CID Scan Start22 bitsOffset (in units of mini-slots) to thestart of the scanning interval from themini-slot boundary specified by thedownlink Allocation_Start_Time Scan Duration22 bitsDuration (in units of mini-slots)where the MSS may scan forneighbor BS}
Referring to Table 6. the SCANNING_IE message includes scan information for use in the SC physical channel. Parameters contained in the SCANNIG_IE message are a CID (Connection ID), a Scan Start value, and a scan duration value. The CID includes an MSS basic CID for use with the SCANNING_IE message. The Scan Start value is a predetermined time at which the MSS begins a pilot SINR scanning operation. The scan duration is a predetermined interval during which the MSS performs the pilot SINR scanning operation. The scan start and scan duration values for use in the SC physical channel are configured in the form of mini-slot units.
TABLE 7For OFDM PHY:SyntaxSizeNotesScanning_IE { CID16 bitsMSS basic CID Scan Start18 bitsIndicate the scanning interval starttime, in units of OFDM symbolduration, relative to the start of thefirst symbol of the PHY PDU(including preamble) where theDL_MAP message is transmitted Scan Duration18 bitsDuration (in units of OFDMsymbols) where the MSS may scan forneighbor BS}
Referring to Table 7, the SCANNING_IE message includes scan information for use in the OFDM physical channel. Parameters contained in the SCANNIG_IE message are a CID (Connection ID), a Scan Start value, and a scan duration value. The CID indicates an MSS basic CID for use with the SCANNING_IE message. The Scan Start value is a predetermined time at which the MSS begins a pilot SINR scanning operation. The scan duration is a predetermined interval during which the MSS performs the pilot SINR scanning operation. The scan start and scan duration values for use in the OFDM physical channel are configured in the form of OFDM-symbol units.
TABLE 8For OFDM PHY:SyntaxSizeNotesScanning_IE { CID16 bitsMSS basic CID Scan Start18 bitsThe offset of the OFDM symbol inwhich the scanning interval starts.Measured in OFDM symbols fromthe time specified by theAllocation_Start_time_Field in theDL_MAP Scan Duration18 bitsDuration (in units of OFDMsymbols) where the MSS may scanfor neighbor BS}
Referring to Table 8, the SCANNING_IE message includes scan information for use in the OFDMA physical channel. Parameters contained in the SCANNIG_IE message are a CID (Connection ID), a Scan Start value, and a scan duration value. The CID includes an MSS basic CID for use with the SCANNING_IE message. The Scan Start value is a predetermined time at which the MSS begins a pilot SINR scanning operation. The scan duration is a predetermined interval during which the MSS performs the pilot SINR scanning operation. The scan start and scan duration values for use in the OFDM physical channel are configured in the form of OFDM-symbol units.
The MSS 400, having received the DL_MAP message including the scanning_IE message, scans pilot SINRs associated with neighbor BSs recognized by the NBR_ADV message according to parameters contained in the SCANNING_IE message at step 417. It should be noted that SINRs of pilot signals transferred from the neighbor BSs and the SINR of the pilot signal transferred from the BS 450 to which the MSS 400 currently belongs are continuously scanned, even though it is not illustrated in FIG. 4.
FIG. 5 is a flow chart illustrating a handover request process of an MSS in a broadband wireless access communication system for use with a conventional OFDM/OFDMA scheme. More specifically, an MSS handover request process for use in the IEEE 802.16e communication system is illustrated in FIG. 5.
Referring to FIG. 5, the BS 550 transmits an NBR_ADV message to the MSS 500 at step 511. The MSS 500, having received the NBR_ADV message, transmits a SCAN_REQ message to the BS 550 when it wishes to scan SINRs of pilot signals transferred from the neighbor BSs at step 513. In this case, the time at which the MSS 500 generates a scan request is not directly associated with the pilot SINR scanning operation, such that its detailed description will herein be omitted. The BS 550, having received the SCAN_REQ message, transmits a DL_MAP message including the SCANNING_IE message (i.e., information to be scanned by the MSS 500) to the MSS 500 at step 515. In association with the neighbor BSs recognized by the NBR_ADV message, the MSS 500, having received the DL_MAP message including the SCANNING_IE message, scans SINRs of pilot signals in response to parameters (i.e., a scan start value and a scan duration) contained in the SCANNING_IE message at step 517. It should be noted that SINRs of pilot signals transferred from the neighbor BSs and the SINR of the pilot signal transferred from the BS 550 to which the MSS 500 currently belongs are continuously scanned, even though it is not illustrated in FIG. 5.
If it is determined that the MSS 500 must change its current active BS to another BS at step 519, after the scanning operations of the SINRs of pilot signals received from the neighbor BSs have been completed, i.e., if it is determined that the MSS 500 must change its current active BS to a new BS, the MSS 500 transmits an MSSHO_REQ (Mobile Subscriber Station HandOver Request) message to the BS 550 at step 521. The MSSHO_REQ message configuration is shown in Table 9 below.
TABLE 9SyntaxSizeNotesMSSHO_REQ Message_Format( ){ Management Message Type=?8 bits Estimated HO time8 bits N_Recommended8 bits For(i=0;j<N_NEIGHBORS;j++){  Neighbor BS-ID48 bits   BS S/(N+I)8 bits }}
Referring to Table 9, the MSSHO_REQ message includes a Management Message Type field identifying a plurality of IEs (i.e., transmission message type information), an estimated HO time field including a handover start time, and an N_Recommended field including the scanning result of the MSS. In this case, the N_Recommended field includes ID information of neighbor BSs and SINR information of pilot signals of the neighbor BSs. It should be noted that the Management Message Type field to which the MSSHO_REQ message will be transmitted is currently in an undecided state, as denoted by “Management Message Type=? (undecided)”.
After transmitting the MSSHO_REQ message to the BS 550, the MSS 500 re-scans SINRs of pilot signals in association with the neighbor BSs at step 523.
First and second problems of the MSS scanning operation for use in the IEEE 802.16e communication system will now be described herein below.
In the first problem, although the MSS scans pilot SINRs of neighbor BSs in response to the scanning information received from the active BS, there is no procedure for additionally reporting the pilot SINR scanning result of the active BS and neighbor BSs. In the second problem, there is no procedure for enabling the MSS to scan pilot SINRs of neighbor BSs before the MSS transmits a scan request to the active BS.
In order to enable the IEEE 802.16e communication system to support a handover function of the MSS, a handover function of a mobile subscriber must be made available upon receipt of a request signal from the MSS and a request signal from the BS. In order to enhance system efficiency, it is desirable that the BS continues to manage the pilot SINR scanning state (i.e., the MSS state) after the MSS has been powered on. However, the IEEE 802.16e communication system cannot report an MSS handover procedure and an MSS pilot SINR scanning state upon receiving a request signal from the BS, such that there must be newly developed such procedures for reporting the MSS handover procedure and the MSS pilot SINR scanning state.